The present invention relates to water borne paint composition, and particularly to water borne paint compositions containing copolyetherpolyols.
Environmental regulations restrict the amount of VOC that a paint applicator can emit, and they also restrict the amount of VOC that a paint can carry. Waterborne paints contain solvents and coalescents that contribute to the total VOC. The presence of solvents reduces the viscosity of a water-soluble resin, such as a water-soluble alkyd, thus helping the paint to flow out into an even film during the early stages of baking. In the case of latexes, a coalescent solvent helps the latex particles to fuse together into an even coating before curing has advanced too far. The most common solvents and coalescents used in water-borne paints are monoethers of ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol. Formulators of water-borne paint need to utilize these volatile organic compounds, but they try to minimize their levels in order to keep the VOC as low as possible. Any way to reduce the levels of those solvents without affecting other properties of the paint or the baked coating would be an advantage.
Binders and pigments account for most of the solids content of the paint. Paint solids can be described in terms of the weight percent solids or of the volume percent. The volume percent solids is more fundamentally related to the thickness of the dried film (dft). For a given volume of paint applied to a given area, the dft depends on the volume percent solids. Throughput on high-speed paint lines is often limited by the solids content. In such cases, throughput could be increased if the paint had higher volume percent solids. However, the volume percent solids cannot simply be increased by removing some of the water, because then the paint would become too viscous to spray. Any way to increase the volume percent solids without causing a concomitant increase in viscosity would be an advantage.
The organic portion of coatings is susceptible to oxidative degradation in the presence of heat and air. Baked enamels are exposed to such conditions during their normal cure, and oxidative degradation is possible during a normal bake cycle or during overbaking. Some resins become yellow during baking by oxidative degradation. Alkyd resins made using unsaturated oils are especially susceptible to such overbake yellowing. Sometimes an alkyd resin can become sufficiently yellow during normal baking so that it is impossible to use that resin to make a very clean white paint. Antioxidants can help reduce the yellowing, but their effectiveness is limited and somewhat costly. Alkyds made using a saturated oil are much more resistant to oxidative yellowing, but they are more expensive than the more common alkyd resins. It would be advantageous to have a binder that is more resistant to oxidative yellowing than common alkyd resins, that could be used to replace some alkyd resin, and that is not much more expensive than an unsaturated alkyd.
It has been proposed to use copolyetherpolyols in water borne paints as dispersing agents for pigments. Those skilled in the art recognize that the function of such copolyetherpolyols is limited to a dispersant function. The prior art has not recognized that such copolyetherpolyols can be used in large quantities so as to become part of the film deposited on a painted substrate during the baking cycle which is not susceptible to oxidative yellowing as so frequently occurs in the case of, for example, alkyd resin films.
It is accordingly an object of the present invention to provide a water borne paint composition which overcomes the foregoing disadvantages.
It is a more specific object of the present invention to provide a water borne paint composition in which use is made of a non-volatile copolyetherpolyol which can replace at least a portion of the conventionally used water-reducible film forming binders to thereby reduce the viscosity of the paint composition as well as reduce its VOC content.
It is yet another object of the invention to provide a water borne paint composition containing copolyetherpolyols in which the copolyetherpolyols are cross-linked with the film forming binder to become part of the film formed during baking of the painted substrate to provide a coating having improved resistance during baking to oxidative yellowing.
These and other objects and advantages of the present invention appear more fully hereinafter from a detailed description of the invention.